Fluorescent lamp



United States Patent FLUORESCENT LAMP Rudolph Nagy and Eugene P. Etzel,Upper Montclair, N. 5., assignors to Westinghouse Electric Corporation,East Pittsburgh, Pa a corporation of Pennsylvania No Drawing.Application June 4, 1951, Serial No. 229,869

'7 Claims. (Cl. 313-409) Our invention relates to fluorescent lamps andmore particularly to binders for the fluorescent powder therein.

The inside surface of the envelope of a fluorescent lamp is coated witha luminescent powder, known as a phosphor, which emits visible lightwhen excited by a source of radiation which is usually within theultraviolet range. In order to effect adherence of the phosphor to thefluorescent lamp, a binder is commonly used with the phosphor coating.Various binders have been developed, but the one most used commerciallyconsists of an organic substance, such as nitro-cellulose dissolved in asolvent, such as butyl acetate. After coating the lamp the organicbinder is burned out by a process known as lehring.

While such organic binders have proved satisfactory in many respects,the fluorescent coatings still chip if the lamp is subjected to roughhandling and mercury tracking may result if the lamp has mercurytherein. Mercury tracking is caused by mercury globules striking thefluorescent coating and knocking it from the lamp envelope. Thistracking problem is experienced chiefly in circular fluorescent lamps.

Therefore, the principal object of our invention is to provide animproved binder for finely divided particles such as phosphors.

Another object of our invention is to provide a binder for fluorescentlamp phosphors which will offer the maximum resistance to chipping andmercury tracking.

A further object of our invention is to improve the binding qualities ofphosphor mixes by adding thereto a small proportion of powdered glass orglass forming materials.

Yet another object of our invention is to provide a glass which whenadded to the binder will not appreciably affect the light output of thelamp, either initially or at some other time during the life of thelamp.

Another object of our invention is to provide a method of applying acoating of luminescent particles to a surface, which coating hasincorporated therein finely divided glass particles to aid in effectingadherence of the particles.

The novel features that we consider characteristic of my invention areset forth with particularity in the appended claims. The invention,itself, however, together with additional objects and advantages thereofmay be best understood from the following description of specificembodiments.

In manufacturing fluorescent lamps, the phosphor is applied to theinside of the lamp by suspending it in a liquid vehicle, coating theinside surface with the suspension, and then evaporating the vehicle. Inorder to increase the adherence of the phosphor to the inside glasssurface, a lacquer containing a binder is added to the vehicle. Thislacquer commonly consists of an organic binder such as nitrocelluloseand a solvent for the binder such as butyl acetate, amyl acetate, orethylacetate. The solvent may also be used as the suspending vehicle forthe phosphor. The final suspension and mixture which is used to coat thelamp is known to thetrade as the phosphor paint. This paint may alsoinclude other agents, as desired, such as one to increase its Wettingproperties or a plasticizer.

We have found that the resistance of phosphor coatings in fluorescentlamps to chipping and mercury tracking may be decidedly improved by theaddition of powdered glass to the binder used in the coating mixture.The glass is preferably a low melting point glass or chemical compoundwhich will melt at lehring temperatures which are usually between 800and 1300 F. In the practice of our invention, the glass may be mixedwith the phosphor before it is suspended in the paint which is used tocoat the inside surface of the fluorescent lamp envelope. The proportionof glass powder is not critical, but the optimum ratio appears to beapproximately one part glass to 99 parts phosphor by weight? Preferablythe glass powder is fine enough to pass a 400 mesh screen. be used.

If preferred, the glass binder may be added to the paint rather thanmixing it with the phosphor prior to its suspension as was done above.This may be done by ballmilling the glass until a fine powder isproduced. The powder is then suspended in an equal volume of binderlacquer and ballmilled once more. The resulting lacquer is mixed withthe phosphor paint in such amounts so as to produce a ratio ofapproximately one part glass to 99 parts phosphor. The percentage ofglass may be varied from .05 to 3 if so desired; however, one per centappears to be optimum.

The following procedure was used in preparing sample lam s for testpurposes. The glass powder was mixed with 150 Kg. of phosphor. Themixture of the phosphor and the glass was then added to 41 liters ofbutyl acetate. To this mixture was added 40 liters of nitrocelluloselacquer, which consisted of nitrocellulose dissolved in butyl acetate;the resultant mix was ballmilled for approximately 5 to 6 hours. Afterthe ballmilling, another 84 liters of the nitrocellulose lacquer wereadded to the mixture; and the resultant product was ballmilled forapproximately a half hour. Finally, an additional 168 liters ofnitrocellulose lacquer were added; and the final product was mixed, asby means of an agitator. Lamps were then coated with the paint andlehred at ordinary lehring temperatures to remove the organic binder byburning in the presence of air.

The composition of the glass is important. In order to make a lampcoated with a phosphor containing an additive of glass commerciallysuccessful, the glass must not adversely affect the output ormaintenance of the lamp. The following are raw mixes of glasses whichhave proved satisfactory.

Example I.A zinc aluminum phosphate glass Raw material ingredients:Parts by weight ZnO 81 Al(OH) 78 H3PO4 (85%) 98 The ingredients arepreferably mixed in a Pyrex beaker and dried at 120-150 C; in an ovenovernight. The dried mixture is then desirably ballmilled for 3 hoursand fired at 1200 C. to produce a very fluid melt. The molten glass ispoured onto a cold surface and ground to a fine powder by the usualmechanical means. The resulting glass has the following composition:

Component: Parts by weight ZnO 81 However, material passing 100 mesh canExample II.A zinc sodium bore-phosphate glass Raw material ingredients:Parts by weight Z110 44.7 Na2C03 1l./ H3303 6.2 H3P04 (85%) 40.2

The glass is prepared as in Example I. The resulting glass has thefollowing composition:

Component: Parts by weight Na20 6.85 B203 3.45 P265 26.4

A complex boric oxide and phosphorus oxide glass having a low meltingpoint and comprising:

Component: Per cent P205 2.87O

R2O3 10 or less R or less R0 15 or less has also proved satisfactory.

Example III Component: Per cent by weight P295 4.95

Lao 5.8

Example IV Component: Per cent by weight P205 9.4

(1210 7.0 LizO 10.5

Example V Component: Per cent by weight P205 50.0

SiOz 1.6

BaO 1.7

ZnO 6.7

Example VI Component: Per cent by weight P205 70.0 B203 5.0 ZnO 4.0 Si02M 5-0 K20 15.0

Example VII Component: Per cent by weight P205 66.8

BaO 1.0

K20 16.6 ZnO 4.5

If more specific formulations are desired, the two following, which arederived from the above general formulation, may be used.

A. A low melting point glass consisting of the following components byweight: approximately 50% to about boric oxide, about 1% to about 15%phosphorous oxide as an additional network former, about 15% to about25% total R0 and R20 content, and 0 to about 10% R203.

B. A low melting point glass consisting of the fol lowing ingredients inthe given proportions by weight; phosphorous oxide, about 50% to about75 SiOz and B203 as an additional network former, about 1% to about 18%;and R0 and R20, about 10% to 24%.

Examples It and 1V fall within the A formulation, and Examples Vi andVII fall with formulation B.

0f the above glasses, the zinc aluminum phosphate is preferable for usein production because it does not fuse as completely as the others thusfacilitating the end cleaning after lehring, and the re-claiming offaulty lamps. The binding action of the z nc aluminum phosphate glassappears to be furnished by a sintering of the glass rather than anactual melting. The ability to re-claim lamps is important in reducingthe shrinkage factor during manufacture. Where re-claiming and endcleaning are not important factors, the other glasses above aresatisfactory since they do not appreciably reduce the lumens per wattoutput of the lamp at either the Zero hour reading or after hours ofuse. Other glasses may possibly be used but they are not preferred dueto low maintenance and initial reduction of the output of the lamp.

The bulbs coated with one of the above glasses added to their coatingmixture, showed increased resistance to mercury tracking and chippedcoating. The bulbs were tested for chipping by placing them on a machinewhich vibrated them at a rapid rate. In lamps not embodying ourinvention, the phosphors rapidly flaked off while this was not true withlamps which had our glass binder therein.

It will be seen from the foregoing that we have provided an improvedbinder for effecting adherence of finely divided particles to supportingsurfaces, as well as a method for applying the binders. When the glassbinder is used with phosphors, they will offer maximum resistance tochipping and mercury tracking without adversely affecting the lightoutput or the maintenance of the lamp.

While we have shown and described a specific embodiment of ourinvention, we are fully aware that many modifications thereof arepossible. We do not intend to be restricted, therefore, except as isnecessitated by the spirit and scope of the appended claims.

We claim:

1. A phosphor coating composition for coating 2. surface comprisingfinely divided particles and a binder to elfect adherence to saidsurface, said binder having therein approximately one part powderedglass for 99 parts of phosphor in said coating, said glass being a zincaluminum phosphate glass.

2. A phosphor coating composition for coating 21 fluorescent lampsurface containing a binder to effect adherence of said phosphor to saidsurface, said binder comprising an organic substance and approximatelyone part glass for 99 parts of phosphor in said coating, said glassbeing a zinc aluminum phosphate glass.

3. A method of producing a phosphor coating on a surface comprisingintimately mixing 1 part of zinc aluminum phosphate glass particles byWeight with 99 parts phosphor particles, suspending the mixture in aliquid vehicle containing an organic binder, applying a coating of thesuspension to a surface, and heating the coating to burn out the organicbinder and sinter the glass particles.

4. A coating composition for applying a phosphor coating to the insidesurface of a fluorescent lamp envelope comprising, a binder, a solvent,and a finely divided phosphor capable of emitting visible light whenexposed to ultraviolet radiation, said solvent being selected from thegroup consisting of butyl acetate, amyl acetate and ethyl acetate, saidbinder consisting of nitrocellulose and finely divided glass, said glassbeing comprised of about 81 parts by weight of zinc oxide, 51 parts byweight of aluminum oxide, and 64 parts by weight of phosphoruspentoxide, and the ratio by weight of said glass to said phosphor insaid coating being from ODS/99.95 to 3/97.

5. A coating composition for applying a phosphor coating to the insidesurface of a fluorescent lamp envelope comprising, a binder, a solvent,and a finely divided phosphor capable of emitting visible light whenexposed to ultraviolet radiation, said solvent being selected from thegroup consisting of butyl acetate, amyl acetate and ethyl acetate, saidbinder consisting of nitrocellulose and finely divided glass, said glassbeing comprised of about 81 parts by weight of zinc oxide, 51 parts byweight of aluminum oxide, and 64 parts by weight of phosphoruspentoxide, and the ratio by weight of said glass to said phosphor insaid coating being about 1 to 99.

6. In a fluorescent lamp, an envelope surface having a coating thereon,said coating consisting of a phosphor and a glass, said phosphor beingcapable of emitting visible light when exposed to ultraviolet radiation,said glass being sintered to said fluorescent lamp envelope surface andbeing comprised of about 81 parts by weight of zinc oxide, 51 parts byweight of aluminum oxide, and 64 parts by weight of phosphoruspentoxide, and the ratio of said glass to said phosphor in said coatingbeing from ODS/99.95 to 3/97.

7. In a fluorescent lamp, an envelope surface having a coating thereon,said coating consisting of a phosphor and a glass, said phosphor beingcapable of emitting visible light when exposed to ultraviolet radiation,said glass being sintered to said fluorescent lamp envelope surface andbeing comprised of about 81 parts by weight of zinc oxide, 51 parts byweight of aluminum oxide, and 64 parts by weight of phosphoruspentoxide, and the ratio by weight of said glass to said phosphor insaid coating being about 1 to 99.

References Cited in the file of this patent UNITED STATES PATENTS2,203,898 Devries June 11, 1940 2,393,469 Hooley Jan. 22, 1946 2,400,147Hooley May 14, 1946

6. IN A FLUORESCENT LAMP, AN ENVELOPE SURFACE HAVING A COATING THEREON,SAID COATING CONSISTING OF A PHOSPHOR AND A GLASS, SAID PHOSPHOR BEINGCAPABLE OF EMITTING VISIBLE LIGHT WHEN EXPOSED TO ULTRAVIOLET RADIATION,SAID GLASS BEING SINTERED TO SAID FLUORESCENT LAMP ENVELOPE SURFACE ANDBEING COMPRISED OF ABOUT 81 PARTS BY WEIGHT OF ZINC OXIDE, 51 PARTS BYWEIGHT OF ALUMINUM OXIDE, AND 64 PARTS BY WEIGHT OF PHOSPHORUSPENTOXIDE, AND THE RATIO OF SAID GLASS TO SAID PHOSPHOR IN SAID COATINGBEING FROM 0.05/99.95 TO 3/97.